In current titanium-sapphire high-intensity laser systems, and more particularly lasers with peak intensity exceeding the terawatt level, the pulse duration has not reached the physical limit yet. As it is common for lower intensity femtosecond lasers to produce few-cycle pulses, i.e. typically less than three optical cycles, the methods used to reach this regime are still not adapted to more energetic lasers which could benefit from a reduction in pulse duration.
In the state of the art, an ultrashort pulse coming out of a titanium-sapphire amplifier undergoes a nonlinear process, which allows further decreasing the pulse duration at the output of the laser system. Typically, a 25 fs pulse with a few mJ of energy at a central wavelength of 800 nm is propagated inside a meter-long hollow fiber filled with a noble gas and the interaction of the laser pulse with the gas creates new spectral components, thus broadening the laser spectrum. Then, by using an appropriate dispersive line to compensate for the delay between the different spectral components, it is possible to obtain pulses as short as 5 fs with up to 5 mJ energy [1]. Another version of this method is to replace the propagation through the noble gas by propagation through a bulk nonlinear medium [2,3], which allows avoiding guidance of the laser beam since the nonlinearity is typically 1000 times more important than for a gas medium.
The hollow fiber method is widely spread but cannot be used for terawatt-class and above femtosecond lasers, since the coupling of such a high intensity laser beam inside the fiber would inherently damage the fiber.
In the case of propagation inside a bulk medium, previous results have shown that the spectral broadening is not constant across the beam profile. The reason is that the laser beam has a Gaussian intensity profile. The nature of the nonlinear process leads to a broader spectrum for the central part of the beam than for the wings of the beam, which is of concern for the quality of the pulse compression. In order to homogenize the spectrum across the beam profile, it was proposed to focus the laser beam and place a spatial filter somewhere close to the focal plane. This solution yields a constant spectrum but the spatial filter damage threshold is still limiting the energy scalability. Another problem is that the spatial filter typically has only 50% throughput, resulting in a loss of half of the energy.